Removed Spam But Topic Remains Current

I run my bbPress forum so that users do not need to register. Moderators log in to post and are the only required registered users. So far Akismet has been fabulous catching spam.

Today Akismet marked a legitimate reply as spam and I quickly unchecked that so it would post properly. No problem there.

Later, a spam reply was missed by Akismet because it appears to have relatively normal text. But I recognized the text to be spam and the username confirmed that. I added the IP to a blacklist and checked it as spam. No problem there.

The problem I have is that the topic the reply was posted under is now appearing at the top of the topic order in the forum indicating that I have just posted a reply recently. The spam is removed so the last date of a reply would have been mine, in December.

Is there anyway to get this topic to appear where it should on the forum page, well below more recent topics?
Thank you, Matoca

Stephen,
Thank you! You truly have an encyclopedic brain. This worked exactly the way it is described. I am going to read up on the options so I have a better idea of what each one does.

Robin, I neglected to add that I do empty the spam each time I check that folder. I collect the IP addresses and make sure the text of each suspect is truly spam. Then I select them all and Empty Spam.
my best, matoca